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The convergence of electron and hole ground states of a dome-shaped In0.6Ga0.4As quantum dot as a function of the size of the surrounding buffer is explored within an sp3d5s* tight binding model. It is found that although the quantum dot encompasses only 2 × 105 atoms, proper convergence of ground state eigenenergies requires that over 10 times as many atoms need to be included in the simulation domain. It is also found that the disorder-induced broadening is very sensitive to the applied boundary conditions. Examination of local eigenenergies as functions of position shows similar convergence problems and indicates that an inaccurate resolution of the equilibrium atomic positions due to truncation of the simulation domain may be the source of the slow ground state convergence.